The present invention pertains to a communication method for transmitting TT ethernet messages in a distributed real time system, comprising a plurality of node computers, wherein each node computer has at least one ethernet controller, which ethernet controller is directly connected via a data line to a port of a TT star coupler clearly assigned to the node computer, and wherein a plurality of TT star couplers may be directly or indirectly connected to one another via one or more data lines in order to form a closed TT network.
Furthermore, the present invention pertains to a TT star coupler for relaying ethernet messages in an above-mentioned communication method.
In the past 20 years, IEEE Ethernet Standard 802.3 has become so widely accepted that the costs for ethernet-based communication systems have decreased very sharply based on the present mass market for ethernet controllers in the field of personal computers. For these cost reasons, ethernet has also been increasingly used in real time data processing. In European Patent EP 1 512 254, a method is disclosed, which makes it possible to transmit time-triggered messages with good real time property in an expanded ethernet system—called TT (time-triggered) ethernet below.
In TT ethernet, a distinction is made between two categories of messages, conventional ethernet messages (called ET (event-triggered) messages below) and new TT messages. TT messages are characterized in that they contain in the ethernet type field a bit pattern (bit pattern 88d7) authorized by the ethernet standard management of IEEE. While ET messages (i.e., conventional ethernet messages) come from a temporally uncoordinated, open environment and hence may be in temporal conflict with one another, it is assumed in TT ethernet that all TT messages can be transmitted without obstructing one another according to an a priori set schedule in a closed TT network. The closed TT network comprises a number of node computers that communicate via one or more TT star couplers.
The useful data efficiency of the transmission of TT messages in TT ethernet depends greatly on the precision II of the clock synchronization of TT ethernet controllers in the node computers, since the time interval between two TT messages must be greater than twice the precision II to be able to rule out any collision of TT messages. A hardware support requires a very precise clock synchronization (approximately in the range of 1 μsec), which is not found in commercially available ethernet controllers. If the clock synchronization is performed in the software, then it is difficult to realize a precision higher than 50 μsec, i.e., a time interval of at least 100 μsec must be scheduled between two TT ethernet messages in order to rule out a collision of TT messages in the TT network. If it is assumed that the transmission time of many TT messages in a 100 Mbit/sec ethernet system is substantially shorter than 100 μsec, then a useful data efficiency of far less than 50% may occur when using commercially available ethernet controllers.